1. Field of the Invention
This invention relates to the field of printed circuit assemblies manufacturing and, in particular, to processes for manufacturing printed circuit boards.
2. Prior Art
Since their discovery, printed circuit techniques have become a commercially important part of almost every area of the electronics industry. Printed circuitry has become virtually the only economically viable technique for the interconnection of components in even fairly low volume production. In high volume production, printed circuitry is universally employed.
Recent technological developments in integrated circuits and their consequent widespread acceptance and use in essentially every phase of consumer, commercial, industrial and military and aerospace electronics have resulted in two conflicting demands being placed upon printed circuit manufacturers. First, the complexity of the interconnection required, coupled with the inherent miniaturization of integrated circuits themselves have led to increased demands for further and further miniaturization of printed circuits in order to accommodate the desired integrated circuit functions within a package which does not become so large that the advantage of miniaturization is lost in the interconnection process. Second, as circuit densities increase in response to the demand for miniaturization and the complexity and number of interconnection likewise increase, the opportunities for failure similarly increase thereby giving rise to a demand for inherently higher reliability interconnection techniques. The response to these demands has been a series of improvements in printed circuit manufacturing processes with consequently higher densities and reliabilities being obtained. Still further improvement is needed, however, in order to allow further miniaturization and reliability improvements.
Of particular concern in the past have been the techniques employed for plated-through holes, and the compatibility of the printed circuits themselves with mass-production soldering equipment such as "wave" soldering. To use plated-through holes as conductors or as component lead receptables it is necessary that a solder-compatible material, preferably solder itself, be plated on the walls of the holes and on adjacent terminal pad areas. To do this realiably, the usual solution has been to apply solder to all areas of the conductor traces. Both the presence of the solder and the method of applying it, however, contribute to a loss of reliability and a limitation on the useful life of the board as well as placing severe restriction on the minimum spaces allowable between adjacent conductors.